Here are some words that could be used interchangeably with "geology," depending on the context:

General:

* Geoscience

* Earth science

* Geophysics

* Geochemistry

* Mineralogy

More specific:

* Petrology (study of rocks)

* Stratigraphy (study of rock layers)

* Paleontology (study of fossils)

* Structural geology (study of the Earth's structure)

* Geomorphology (study of landforms)

* Hydrogeology (study of groundwater)

The best word to use will depend on the specific aspect of geology you are referring to.